1. Field of the Invention
The present invention relates generally to interconnects for integrated circuits and more particularly to the fabrication of Schottky and ohmic contacts to a silicon substrate using aluminum and silicon doped aluminum.
2. Prior Art
A major product of the semiconductor industry is integrated circuits formed of planar semiconductor devices in a substrate of silicon. Generally, a silicon oxide or glass coating overlies the silicon substrate, except in the actual contact areas. This coating functions to pacify the junctions and provide an insulation between the devices and the interconnects. Accordingly, the contact metal material must exhibit good adherence to the silicon and to the silicon oxide or glass, while not producing any undesirable reaction with--nor penetration of--the silicon or the oxide.
A metal interconnect system for integrated circuits must make a low resistance, non-rectifying contact to all types of silicon and must not react substantially with the silicon at moderate temperatures (500.degree. C.). Aluminum has been found most suitable for use as interconnect with silicon planar devices in integrated circuits. Aluminum is an excellent conductor and adheres well to silicon and silicon oxide. It is also easily applied to semiconductor devices by evaporation and photoresist techniques and contacts made of aluminum are readily bondable with gold or aluminum wires.
Though aluminum has many excellent qualities and is widely used in the industry, it also has a variety of disadvantages. Specifically, aluminum and silicon greatly interdiffuse at 500.degree. C. and since aluminum is a P type dopant, it can form a PN rectifying junction with the N type silicon. Thus, aluminum does not make a good ohmic contact with lightly doped N type silicon.
In a high frequency double-diffused NPN transistor, the emitter region is typically diffused into the base region to a very shallow depth of only about 1,500 to 2,000 angstroms. Due to the small geometry of the shallow double-diffused transistor, the edge of the emitter diffusion opening in the oxide mask layer is so close to the emitter-base junction at the surface of the wafer that horizontal migration of the interaction between the aluminum and the silicon formed during the alloying process of the aluminum at approximately 500.degree. C. often shorts out the base-emitter junction as well as vertical migration, which may also short out the emitter-base junction.
One of the prior art's solutions is to use different contact metals for different contact regions. In U.S. Pat. No. 3,946,426, for example, contacts include molybdenum engaging all contact areas of N conductivity type regions and aluminum engaging said molybdenum and all contact areas of P conductivity type regions. Another prior art solution to the aluminum pitting or alloying with the silicon surface is to use a silicon doped aluminum contact metal. This is described in U.S. Pat. No. 3,382,568. Both of these solutions prevent the formation of Schottky contacts with N type regions. Although it is well known to use aluminum to form a Schottky contact with lightly doped N conductivity type silicon regions, as illustrated in U.S. Pat. Nos. 3,909,837 and 4,005,469, the pitting and alloying problem of aluminum with these regions is not addressed. Thus there exists a need for a metal interconnect system which is capable of forming Schottky and ohmic contacts with shallow N conductivity type silicon regions.